The β-amyloid peptide (Aβ) is the major constituent of the neuritic plaques that together with the neurofibrillar tangles are physiologic hallmarks of Alzheimer's disease (AD). Aβ, a 39-43 amino acid peptide, is derived from proteolytic cleavage of the amyloid precursor protein (APP), a type I integral membrane protein that is expressed ubiquitously. APP can be processed via alternative pathways; a non-amyloidogenic secretory pathway includes cleavage of APP to soluble APP (sAPP) by a putative α secretase within the sequence of Aβ peptide, thus precluding the formation of Aβ, whereas the formation of the amyloidogenic Aβ peptides is regulated by the sequential action of β- and γ-secretases. As the proportion of APP, processed by β-secretase vs. α-secretase, may affect the amount of Aβ produced, the regulation of these two pathways may be critically important to the pathogenesis of AD. Proteolyic processing of APP can be regulated by activation of various cell-surface receptors coupled to increased activation of second messenger cascades, including phosphatidylinositol hydrolysis, tyrosine phosphorylation, protein kinase C (PKC), protein kinase A, mitogen-activated protein kinase, protein phosphatase 1 and 2B, and calcium.
It has been suggested previously that Aβ toxicity and aggregation involve transition metals, oxidative stress, and accumulation of reactive oxygen species (ROS). It was also shown that Aβ neurotoxicity might be attenuated by numerous antioxidants and iron chelators. Recently, an increasing body of evidence points to a wide array of non-vitamin antioxidants, such as polyphenolic compounds, that can scavenge ROS, chelate transition metals, such as iron and copper and protect cells from oxidative damage. Indeed, (−)-epigallocatechin-3-gallate (EGCG), the major polyphenol isolated from green tea, was shown in several studies to possess neuroprotective effects against a variety of toxic insults and neuronal injuries.
Over the past decade, intense focus has been given to investigating the processes of APP proteolysis and AP metabolism as possible targets for AD therapy (Hardy and Selkoe, 2002). Various synthetic and naturally occurring compounds have been analyzed for their efficacy in the modulation of these pathological events. One such naturally occurring compound achieving worldwide popularity for its therapeutic application is green tea. Green tea contains polyphenolic structures categorized as flavonoids, which are believed to be the active components accounting for the therapeutic properties of green tea. Arguably, one of the most promising green tea compounds being analyzed is (−)-epigallocatechin-3-gallate (EGCG), which has been extensively studied primarily because of its reported anticarcinogenic effects (Lin and Liang, 2000 Moyers and Kumar, 2004). Recently, EGCG has been found to modulate protein kinase C (PKC) activity and to consequently increase secreted levels of sAPP-α (Levites et al., 2002; Levites et al., 2003). Additionally, EGCG has been shown to inhibit various activities of proinflammatory cytokines (Ahmed et al., 2002; Han, 2003; Li et al., 2004). Accordingly, signal transducer and activator of transcription 1 and nuclear factor kB responses are inhibited by EGCG (Han, 2003; Aktas et al., 2004). Elucidation of these molecular actions of EGCG substantiates the compound as a versatile modulator of cellular responses that may contribute to disease pathogenesis.
EGCG treatment leads to a significant reduction in Aβ production as well as decreased Aβ levels and β-amyloid plaques in the brain. These effects are associated with increased generation of α-CTF and sAPP-α and elevated α-secretase cleavage activity, showing that EGCG promotes the nonamyloidogenic α-secretase proteolytic pathway both in vitro and in vivo.
As shown above, epigallocatechin gallate (EGCG) has recently gained the attention of scientists for implementation as a therapeutic agent for the treatment of many diseases. Current research suggests that EGCG has implications as a treatment for Alzheimer's disease and HIV-associated dementia (HAD). The full benefit of EGCG, however, has yet to be fully realized due to its low bioavailability in vivo.
It has been shown that an oral dose of 800 mg/70 kg/day provides approximately 400 ng/ml EGCG in human plasma. The inventors have recently shown that the dose of EGCG (1000-2000 ng/mL) is necessary for promoting APP α-secretase cleavage in SweAPP N2a cells. Using linear approximation, in order to reach plasma concentrations of 1000 ng/ml, an oral dose of about 1800 mg/70 kg/day would be required. From a safety point of view, this dose might be unacceptable for clinical trials. Since the oral EGCG dosage in most clinical trials for cancer therapy is typically not more than 800 mg/day, regimens which enhance EGCG bioavailability, effecting reductions in Alzheimer pathology and cognitive decline at minimum doses of EGCG, are very desirable. Thus, the absolute bioavailability of EGCG is an important issue for oral administration of EGCG to AD clinical trial. It has been previously reported that decreased bioavailability of EGCG is greatly associated with the glucuronidated form, which is largely present in the plasma of treated mice. Recently, these same researchers further showed that piperine, an alkaloid derived from black pepper, enhances the bioavailability of EGCG by inhibiting glucuronidation. Unfortunately the consumption of piperine for enhancing for EGCG bioavailability would likely be precluded by the side-effects conferred by piperine. Thus what is needed is a method and composition of increasing the bioavailability of EGCG that does not need to be administrated with other compounds and does not confer any unwanted side effects on the patient.